Approximately 20000 people die from septic shock every year alone in this country as an outcome of gram negative bacterial infection. Lipopolysaccharide (LPS) or endotoxin, a structural component of outer cell wall, is a major effector and can activate the innate immune system via Toll-like receptors (TLRs) and lead to multiple organ damage, termed endotoxin shock. Human bodies are cohabitating with commensal bacterial flora. The largest observed is the one in the intestinal tract which is approximately 1 kg of bacteria/adult of weight. These bacterial flora continuously yield large amounts of bacterial products such as LPS. It has been shown that previous exposure to bacterial products change the sensitivity to LPS challenge in vitro and in vivo. Since LPS produced by commensal bacterial flora in the intestine is actually observed in blood circulation in the portal vein in laboratory animals, it is conceivable that commensal bacterial flora simulate the immune system, leading to the change of sensitivity to endotoxin shock via TLRs. We will test if commensal bacterial flora alter TLR signaling and sensitivity to endotoxin shock in vivo, using mutant strains of mice with increased TLR signaling and decreased TLR signaling. This research is performed primarily in the Institute of Microbiology AS CR in the Czech Republic in collaboration with Helena Tlaskalova as an extension of NIH grant # P01 AI36529.